If your AC is humming but not cooling, or the outdoor fan won’t spin, a bad capacitor is the most likely cause. It’s the single most common summer AC failure we see. Give us a call and we’ll get you on the schedule as fast as we can.
What a Capacitor Actually Does
The capacitor is a small cylindrical component inside your outdoor unit. It stores and releases electrical energy to start the compressor and fan motors, and to keep them running under load. When it fails, those motors either can’t start at all or run poorly enough that the system shuts down.
There are two types: a start capacitor (a high-microfarad cap that gives the motor a boost each time it starts, then disengages once it’s up to speed) and a run capacitor (stays in the circuit continuously). Many modern units use a single “dual-run” capacitor that handles both the compressor and the fan motor at once.
Symptoms That Point to a Bad Capacitor
The clearest sign is a humming outdoor unit where the fan isn’t spinning. You can sometimes hear the compressor trying to start, making a low grunt before the breaker trips or the unit shuts itself off.
Other things I see regularly:
- The outdoor fan starts only if you give it a gentle push through the grille. That tells you the motor can spin freely but isn’t getting the starting torque it needs, which points directly to the capacitor. Don’t do this deliberately, but homeowners sometimes report the unit “works if I nudge it” after doing it by accident.
- AC runs but struggles to cool, especially on hot afternoons when load is highest
- The unit starts and stops in short cycles
- A noticeable hum with no airflow from the outdoor unit
None of these are exclusive to a capacitor. A seized fan motor, a bad contactor, or a refrigerant issue can look similar from outside. But capacitor failure matches this symptom pattern more often than anything else.
How We Diagnose It
A tech puts a multimeter in capacitance mode across the capacitor terminals and reads the microfarad (µF) value. Every capacitor has a rated value printed on the label, along with a tolerance (typically ±5% to ±6%). If the measured value is outside that range, it’s failing or failed. A reading of zero means it’s already gone.
Visual clues help too: a bad capacitor often bulges at the top or shows a small rupture. That’s the electrolytic fluid inside expanding from heat stress. If you see a swollen top on the capacitor, it’s done regardless of what the meter reads.
The test takes about two minutes once the panel is open and the unit is powered down. That’s the diagnostic, not a teardown.
Why Capacitors Fail
Heat is the main culprit. Capacitors are rated for a certain number of operating hours at a given temperature. In the Bay Area, we run AC hard through July and August, and the outdoor units sit in direct sun. A capacitor that’s been through five or six summers is living on borrowed time.
Power fluctuations can accelerate the process. If your area has had brownouts or if your unit gets cycled on and off frequently by a thermostat set too aggressively, that adds wear. Age is the simplest predictor: most capacitors in residential units last anywhere from 5 to 20 years, with 10 years being a reasonable average under normal conditions.
A capacitor can also fail early if the system has been running with another underlying problem, like a refrigerant shortage that causes the compressor to labor, or a dirty condenser coil that raises discharge temperatures.
What You Can Check
You can visually inspect the outdoor unit through the grille without opening anything. Look for the capacitor (cylindrical, silver or gray, usually mounted near the contactor). If the top is bulging or you see any residue around the case, you have your answer.
Beyond that, leave it. Capacitors hold a charge even after the unit is powered off, sometimes for hours. At 370 to 440 volts, a discharge to the hand is serious. Safely discharging and replacing a capacitor requires knowing the procedure and having the right tools. It’s not a task to learn from a video standing next to live voltage.
Call a Tech
If you’ve spotted the symptoms above or confirmed a bulging cap through the grille, a service call is the right next step. A tech confirms the reading, replaces the part, and runs a full system check in the same visit.
That last part matters. A capacitor can fail early when something else is driving it: low refrigerant, a dirty condenser coil, a motor drawing too much current. Replacing the cap without finding the cause means you’ll burn through another one in a season or two.
If your unit is more than 10 years old and this is the first capacitor failure, it’s also worth a straight answer on repair versus replacement. A service visit is the right time for that conversation.
Replacement Cost
The part itself runs roughly $10 to $50 at supply houses, depending on the type and ratings (dual-run caps tend to run higher than single-run). The labor and service call are where the cost sits. Total repair cost varies by your area, the system type, and what else the tech finds while they’re there. Get a written quote before authorizing work.
If your outdoor unit is humming or the fan won’t spin, give us a call. We’ll get you on the schedule as fast as we can for most Tri-Valley and East Bay locations. You can book or ask a question at adriumservice.com.